Anthropic's first new API version since 2023 removes temperature and top_p

Anthropic released a new Claude API version, 2026-10-21, on October 21, 2026. It is the first new version since 2023-06-01, according to Anthropic’s version history. The same day, Anthropic shipped new major versions of its SDKs and the ant CLI that send 2026-10-21 by default. The biggest change for most apps: a request that includes temperature, top_p or top_k now returns a 400 error, “even at the parameter’s default value.”

What changed

In the Messages API under 2026-10-21:

• temperature, top_p and top_k are removed, on every model.
• A document block placed directly in an assistant message returns a 400 error.
• A text block with a non-empty citations array in a user message returns a 400 error.

The Admin API and the Claude Enterprise Analytics API also drop several old field names, such as workspace_id on API keys, in favor of new ones.

The new SDK versions are Python 2.0.0, TypeScript 1.0.0, C# 13.0.0, Go 2.0.0, Java 3.0.0, PHP 1.0.0, Ruby 2.0.0 and ant CLI 2.0.0. They also have breaking code changes that don’t depend on the API version. And they move from the MIT License to Apache 2.0; earlier versions stay under MIT.

Who it affects

Anyone who upgrades to the new SDK majors, or sets the new header by hand. Older SDKs keep sending 2023-06-01, and the migration guide tells developers who rely on a non-default temperature to “keep that request on 2023-06-01.” Anthropic’s pages give no retirement date for the old version.

The practical effect depends on the model. Anthropic says Claude 4.7 and later models already accept only the default sampling settings, so they behave as before. On older models that take other values, such as Claude Sonnet 4.6, a request that lowered temperature for more consistent output loses that effect, and “no other setting replaces it.” Requests without these parameters sample at the defaults, including a temperature of 1.0.

What to do

1. Search your code for temperature, top_p and top_k in Claude requests and remove them before upgrading an SDK.
2. Move any document blocks out of assistant messages, and drop citations from user text blocks.
3. Read the language-specific SDK changes in the migration guide, and update license records if you track dependency licenses.

To compare Claude’s prices with other models, see our AI model price comparison.